The present embodiments relate to a method and apparatus for determining a beam parameter of an unflattened photon beam generated by an accelerator.
In medical engineering, accelerators are used to generate beams. Linear accelerators (e.g., linac) are typically used, generating electrons or, as secondary or deceleration radiation, photons for the beam used. The generated beam can be used for both diagnostic and therapeutic purposes.
The beam intensity of the generated beam is generally not regular. To achieve a regular intensity over the cross section of the beam used, flattening filters are used. The flattening filters are tailored to the characteristic intensity distribution of the beam so that a regular beam intensity is achieved over the cross-sectional region of the beam due to radiation or photon absorption. Since the unflattened beam generally produces the maximum intensity in the center of the beam and has an intensity characteristic that diminishes continuously with distance from the center, flattening filters may be configured so that the filters have the greatest absorption in the center of the beam, and absorption capacity decreases with distance from the center of the beam. Such flattening filters are disclosed, for example, in EP 0253 046 A1 (e.g., for therapeutic applications) and US 2006/0256925 A1 (e.g., for diagnostic applications).
Attempts have been made during the therapeutic deployment of accelerators for photon generation to dispense with the flattening filter during radiation. Unflattened beams have a considerably higher dose rate than flattened beams. The flattened beam typically has a maximum of 500 monitor units (MU) per minute, with the unflattened beam having a significantly higher range with respect to maximum value (e.g., currently up to 2000 MU/min). “Monitor units” is an internal designation for the output of a linac. Calibration takes place, such that in reference conditions (e.g., defined distance, measuring depth, and field size) an MU corresponds to the dose of 0.01 Gy (Gray). Since very high dose rates (e.g., several thousand MU) are used in applications such as stereotaxy and intensity modulated radiotherapy (IMRT), for example, treatment time is significantly reduced when unflattened beams are used. Fields of smaller extension (e.g., field size) are used for stereotaxy and IMRT. With the standard radiation output of flattened beams, the time required to apply a quantity of radiation for the treatment would therefore be considerable. The time to apply the quantity of radiation is significantly reduced by the increase in radiation intensity associated with the removal of the flattening filter.
The use of unflattened photon beams for therapeutic purposes is also described in Bayouth J. E. et al. “Image-guided stereotactic radiosurgery using a specially designed high-dose-rate LINAC.” Medical Dosimetry. 32.2 (2007): 134-41 and Pönisch F. et al. “Properties of unflattened photon beams shaped by a multileaf collimator.” Med. Phys. 33.6 (2006) 1738-46. Bayouth et al. argue that the use of unflattened beams also results in a reduction in the overall quantity of radiation absorbed by the patient (see p. 136).
Unflattened beams may be described or classified for use. Parameters for beam description have been defined for flattened beams and are used universally in accelerator technology. These parameters are, for example, the parameters field size, penumbra, symmetry and flatness. Since the definition of such parameters was stipulated for flattened beams, the values obtained according to the definitions of the parameters do not provide the same information content for unflattened beams as for flattened beams. Using these parameter definitions for unflattened beams results in parameter values of unflattened and flattened beams no longer being compared at least to some degree. Parameter values of unflattened beams are also no longer directly comparable with a different beam extension. A complex conversion may be used for comparison, as is attempted, for example, by F. Pönisch et al. in “Properties of unflattened photon beams shaped by a multileaf collimator.”